The invention relates to a method for forming or shaping at least one work piece using high liquid pressure and an apparatus for forming at least one workpiece using high liquid pressure by means of a forming die having an upper part and a lower part, which fix the workpiece in marginal areas, but whilst permitting a material flow.
Hydraulic or high pressure forming was developed with respect to closed workpieces such as pipes. It has also already been proposed to double sheet metal blanks by a circumferential, tight weld and to hydraulically form same by pressurizing in the gap. If the sheet metal blanks are not to be jointly used, it is necessary for them to be subsequently separated again in a complicated manner. A further disadvantage results from the fact that there is a restricted material flow behaviour due to the weld and associated therewith almost the same material, flow for the upper and lower workpiece, which once again significantly limits the choice of part pairings for parts having different geometries or also the possible complexity of the parts. It has therefore been proposed to shape doubled sheer metal blanks without a fixed (welded) connection, the tightness or seal in the marginal area not being brought about by welding, but by subjecting the marginal areas of the two sheet metal blanks to a combined clamping and forming die action, cf. DE 195 35 170 Al. This causes the problem that with a fixed clamping adequate for achieving the necessary tightness, it is not possible to ensure an adequate plastic flow, which is to be made possible by the lack of a welded connection. Although such a plastic flow is permitted by reducing the clamping force, this decreases the tightness between the marginal areas of the sheet metal parts.
Thus, whilst avoiding the aforementioned disadvantages, the problem of the invention is to provide an apparatus for forming workpieces, such as in particular sheet metal blanks, by means of which an optimum forming of optionally also one workpiece is possible.
According to the invention, the set problem is solved with a method of the aforementioned type, which is characterized in that during the pressure build-up, the workpiece is marginally clamped in per se known manner and that in the final phase of forming to the desired final shape of the workpiece (sizing) the marginal area of the workpiece is freed or released.
For solving the set problem an apparatus according to the invention is characterized by a pressure casing surrounding and separated from the forming die.
As a result of the solution according to the invention on pressurizing on one side of a workpiece (or between two workpieces) the necessary plastic flow of the workpieces is ensured, whilst during the final forming or shaping (sizing), the workpieces are marginally exposed and no longer have to be fixed, so that the complete workpiece shape can undergo the sizing process. As intentionally a maximum, high pressure-tight seal in the clamping area of the dies is renounced, the tightness is brought about by a pressure-tight encapsulation of the die, particularly by a pressure casing separate from and completely surrounding the forming die.
In a preferred development, the closing force of the die is adjustable. The forming die closing force can be adjusted in such a way that it is just below the opening force of the sizing pressure necessary for the final forming or sizing of the particular workpieces. So that a controlled release of the clamping action is possible, according to further developments of the invention drive mechanisms for moving die parts on the one hand and casing parts on the other are separate mechanisms and that there is a limited vertical clearance with the die closed, including spacing rings and plates, as well as workpieces within the pressure casing and that the drive mechanism (press mechanism) for the casing has a higher closing force than the drive mechanism for the die, so that the workpieces are clamped in the marginal area by the die with a closing force which is below the closing force with which the casing parts are kept closed.
According to a further development of the inventive method, both workpieces are initially marginally clamped, whilst interposing a spacing element.
According to a particularly preferred development of the inventive apparatus one part (lower part) of the pressure casing has on the inside a circumferential recess and that another part (upper part) of the pressure casing has a projection of limited thickness engaging in the recess, which creates a high pressure seal. Under a high liquid pressure acting thereon within the pressure casing, the projection is pressed radially outwards against the wall or shoulder of the recess on the other part of the pressure casing, so that the tightness is increased with an increasing internal pressure.
In addition, a low pressure seal can be provided. This is brought about by a seal between facing faces of the pressure casing parts. In a specific development, in facing faces of the pressure casing parts are formed grooves in which the seal engages. The low pressure seal can be constructed in different ways either by being constituted by solid, elastic material or a hose fillable with a pressure medium or fluid.
According to another preferred development of the invention, , between the upper and lower parts of the forming die is placed a spacing ring, so that a workpiece can be clamped between part of the die and the spacing ring and optionally a further workpiece is clampable between the spacing ring and the other part of the die. Such a spacing ring leads to the workpieces being desirably mechanically deformable in their die-clamped marginal areas and, if simultaneously two workpieces are shaped, do not have to be shaped in their marginal areas in the same or a complimentary manner. Thus, in a preferred development the top and bottom of the spacing ring is at least in part non-parallel and/or non-planar. Moreover, through a planned introduction of drawing creases or shut-off edges between the spacing ring and die parts the material flow can be controlled differently between the upper and lower workpieces.
According to a further development the spacing ring has a filling inlet. Thus, between directly engaging sheet metal blanks there is no need to provide an inlet for the hydraulic fluid and in said inlet area the sheet metal blanks are shaped in a manner not corresponding to their final shape but due to the necessity of the inlet. In the case of an inlet directly between the edges of sheet metal blanks, as is the case in the prior art, sealing or tightness problems arise. These disadvantages are avoided by the inlet guided substantially radially or laterally through the spacing ring. The inlet can either be directly connected to the inner area of the pressure casing surrounding the forming die or the filling inlet of the spacing ring can be aligned with a filling passage of the pressure casing when the latter and the die are closed. A bushing forms a tight connection between the spacing ring inlet and the pressure casing passage. The bushing producing the connection between the pressure casing passage and the spacing ring inlet is preferably firmly connected to the spacing ring or constructed in one piece thereon and engages with its end facing the casing passage and which is preferably constructed as a circumferential collar in an annular groove of the casing, which can be formed by two semicircular grooves in the upper and lower part at the interfaces of said two parts.
According to another preferred development of the invention there is a closable emptying outlet (through the pressure casing wall). Accompanied by the interposing of spacing plates the die can be located in the pressure casing.